In a typical RF communication system, a transmitted signal may travel from a transmitter to a receiver over multiple paths, for example, a direct path and also a reflected path. The signal on the reflected path typically arrives later than the signal on the direct path. Thus, the received signal exhibits distortion due to the time dispersive nature of a channel. Channel environments such as this are also known as multipath fading channels.
In digital advance mobile phone systems (DAMPS), an equalizer or demodulator typically operates based on the assumption that the transmitted signal encounters a symbol-spaced, two-tap multipath channel, regardless of the actual prevailing channel conditions. In order to demodulate the received signal, it is necessary to synchronize the receiver to a known synchronization sequence contained in the signal. This can be done initially by correlating the received waveform against a local version of the synch word. The synchronization point in a stream of oversampled received data is chosen which maximizes the sum of two points, one associated with each tap of the channel estimate, of a squared correlation taken at two different lags taken a symbol interval apart. By choosing the synchronization point that maximizes the sum, the correlated received power at symbol taps is maximized. This matches the symbol-spaced two-tap channel estimate in time to the actual channel in a maximum power sense.
When rapid fading is present, actual channel conditions at the beginning of a slot burst may substantially change over the transmission of that burst. The conditions for matching the channel estimate in time to the channel in a maximum power sense may change substantially as well. This can result in the optimum synchronization point shifting over the course of the burst. If the same symbol sampling phase is used, then the channel estimate becomes no longer matched in time to the actual channel in that maximum power sense.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.